1. Field of the Invention
The present invention relates to an image signal recording and reproducing system for recording an image signal on a recording medium and for reproducing a recorded image signal from the recording medium.
2. Description of the Related Art
Still video (SV) systems are known as one type of system for recording and reproducing still image signals. The SV systems are commonly arranged to frequency-modulate and record television (TV) signals of an existing format on 2-inch magnetic disks. However, the resolution attained with such a system is limited to that of an existing TV system. For this reason, it is pointed out that, if a printer is used to produce a final printout of a still image recorded by the SV system, the obtained image quality (particularly, the resolution) will be low compared to that of a typical silver-salt photograph.
It is also known that several novel television systems, such as a high-definition television (HDTV) system, have recently been proposed. The HDTV system has approximately one thousand scanning lines which are about twice the number of scanning lines used in an existing NTSC system, and also has a horizontal signal band which can accommodate such a large number of scanning lines. Accordingly, in the field of SV systems, it has been strongly desired to develop a still image recording and reproducing system capable of recording and reproducing a still image signal which corresponds to the image quality of 1,000.times.1,000 pixels (per square image area on a TV screen) realized by the HDTV system or the like.
In light of the above-described circumstances, it has been proposed to provide an SV system which adopts a high-band (wide-band) recording format relative to a recording medium. However, it is necessary to improve the image quality of the SV system while retaining compatibility with conventional systems.
One method of improving the image quality while retaining compatibility with conventional systems is what is called a CHSV (compatible high-definition SV) system.
The outline of the CHSV system will be described below.
The CHSV system utilizes an art called analog transmission of sample values.
A system for analog transmission of sample values is, as shown in FIG. 1, characterized by a transmission-path characteristic (LPF characteristic) and re-sampling. More specifically, the system is arranged so that the input sample value is restored by re-sampling after being passed through a frequency modulation section, an electromagnetic conversion section and a frequency demodulation section.
The principle of the analog transmission of sample values is explained in more detail with reference to FIGS. 2(a) to 2(f). In the following explanation, it is assumed that a sequence of sample values of period T, shown in FIG. 2(a), is recorded and reproduced. The transmission path which includes the frequency modulation section, the frequency demodulation section and the electromagnetic conversion section has a low-band transmission characteristic, i.e., a low-pass filter (LPF) characteristic. FIG. 2(b) shows the output of this transmission path. If the illustrated transmission-path output is re-sampled by means of a sequence of re-sampling pulses which has period T and correct phase, as shown in FIG. 2(c), the output signal shown in FIG. 2(d) is provided. As can be seen from FIG. 2(d), the sequence of input sample values is correctly reproduced (transmitted). However, if the re-sampling pulses are out of phase as shown in FIG. 2(e), the sequence of sample values is not correctly reproduced (transmitted), resulting in ringing such as that shown in FIG. 2(f). Accordingly, to accomplish the above-described analog transmission of sample values, during reproduction (on a receiving side) it is necessary to generate a sequence of re-sampling pulses of correct frequency (period) corresponding to the reproduced (received) sample-value signals, and it is also necessary to generate a sequence of re-sampling pulses of correct phase corresponding to the reproduced (received) sample-value signals.
The other requirement for completely transmitting sample-value signals is as follows: the transmission path, including the frequency modulation and demodulation sections and the electromagnetic conversion section, has a linear phase and a frequency characteristic which is point-symmetrical about a sampling frequency fs/2(=1/2).
More specifically, it is necessary that the transmission path have an LPF characteristic such as that shown in FIG. 3. The outline of the analog transmission of sample values has been explained in brief.
The following is an explanation of a method of recording a luminance (Y) signal on the basis of the CHSV system.
FIG. 4 is a diagram showing sample points for a Y signal to be recorded on a magnetic disk. As shown in FIG. 4, the sample points for a Y signal are arranged in an offset manner, and are subjected to sub-sampling transmission. Also, 650(=1300/2) sample points are present in each row, while 500(=1000/2) sampling points are present in each column. The sample values contained in rows A1, A2 . . . are recorded on a signal track on the magnetic disk, the sample values contained in rows B1, B2 . . . on another track, and so on. In this manner, all the sample points are recorded on a total of four tracks.
The sample points are recorded on each track in a format according to a known SV format. FIG. 5 shows the frequency allocation of a signal recorded in the SV format. As shown in FIG. 5, in the SV format, the basebands of recorded Y and C signals are approximately 6.5 MHz or less and approximately 1 MHz or less, respectively.
In FIG. 4, each row includes 650 Y-signal sample points, and these points are recorded on the portion of an NTSC-TV signal which corresponds to a horizontal available picture interval (53 .mu.sec or less). Accordingly, a corresponding sampling frequency fs (refer to FIG. 3) is approximately 13 MHz or less. In the above-described manner, the Y signal having the band shown in FIG. 3 is recorded.
FIGS. 6(a) and 6(b) show two different recording patterns formed on the magnetic disk on the basis of the CHSV system. FIG. 6(a) shows the recording pattern formed when heads for two channels (2-ch heads) are utilized, while FIG. 6(b) shows the recording pattern formed when heads for four channels (4-ch heads) are utilized. (Needless to say, the 4-ch heads can be utilized to form either of the recording patterns shown in FIGS. 6(a) and 6(b).)
The recording pattern of FIG. 6(a) is formed in the following manner. First, the sample values of Y signals on rows Ai and Bi (i=a positive integer) are simultaneously recorded on first and second tracks, respectively, by means of the 2-ch heads. Then, the 2-ch heads are moved to third and fourth tracks (this movement is not needed when 4-ch heads are in use), and the sample values of Y signals on rows Di and Ci are simultaneously recorded. In this process, to retain compatibility with conventional SV formats, the arrangement of the tracks on which the sample values of the Y signals on the rows D.sub.i and Ci are recorded is reversed as shown in FIG. 6(a).
In general, if two tracks of signals are simultaneously recorded by using 2-ch heads, crosstalk arises between the signals during the recording operation of the two heads. However, with the above-described recording method, what is called H alignment can be implemented; that is to say, two kinds of signals are simultaneously recorded by the two heads with horizontal synchronization established between the recorded signals. Accordingly, deterioration of a reproduced image due to crosstalk does not easily occur in a reproducing operation.
If 4-ch heads are employed, the recording shown in FIG. 6(b) may be performed. More specifically, the sample values of the Y signals on the rows Ai and Bi are simultaneously recorded on the first and third tracks, respectively. Then, the sample values of the Y signals on the rows Ci and Di are simultaneously recorded on the second and fourth tracks, respectively.
With the above-described recording method, in the case of the recording pattern of FIG. 6(a), it is possible to reproduce a frame image based on a conventional SV format from the second and third tracks. In the case of the recording pattern of FIG. 6(b), it is possible to reproduce a frame image based on the conventional SV format from the first and second tracks or the third and fourth tracks. A field image can also be reproduced from an arbitrary track.
The process of recording Y signals in the CHSV system is as described above.
The following is an explanation of the process of recording color-difference line-sequential (C) signals in the CHSV system.
FIGS. 7(a), 7(b) and 7(c) show the relationship between the recording sample patterns of a Y signal, a CR (=R-Y) signal and a CB (=B-Y) signal. In the conventional SV format, the recording band of a color-difference signal is approximately one sixth that of a Y signal, and the color-difference signal is recorded after being converted into a line-sequential signal. Accordingly, the sample patterns of the color-difference signals CR and CB recorded in the CHSV system are as shown in FIGS. 7(b) and 7(c), respectively. On the right-hand side of each of FIGS. 7(b) and 7(c), symbols Ai, Bi, Ci and Di denote lines of Y signals to be recorded on identical tracks, respectively. Although the lines of Y signals do not completely coincide with the lines of corresponding C signals, this partial discrepancy is intended for compatibility with the SV format.
FIG. 8 is a table which shows the relationship between the recording positions of the Y and C signals. In the table, "FIRST STEP" indicates "simultaneous 2-ch recording executed in a first step," and "SECOND STEP" likewise indicates "simultaneous 2-ch recording executed in a second step." As described above, in the first step, recording on tracks 1 and 2 is executed and, in the second step, recording on tracks 3 and 4 is executed. Referring to FIG. 8, for example, in the first step, Y(Ai) and CR(Ai)/CB(Ai) are recorded on the track 1. Y(Ai) indicates a Y signal consisting of a sequence of Y sample values on the line Ai shown in FIG. 7(a) and CR(Ai)/CB(Ai) indicates a color-difference line-sequential signal which starts with a CR signal and which is formed by the CR signal consisting of a sequence of CR sample values on the line Ai shown in FIG. 7(b) and a CB signal consisting of a sequence of CB sample values on the line Bi shown in FIG. 7(c). In FIG. 8, imaging-section outputs Y1, Y2, R, B are signals which are simultaneously outputted from the imaging section of a CHSV camera, which will be described later.
As described above, in the CHSV system, by combining offset sub-sampling and the analog transmission of sample values, information, the amount of which is apparently four times that of information transmitted by the conventional system, can be transmitted over a limited transmission space (twice that of an NTSC system).
Regarding the sampling of luminance signals in the CHSV camera in the above-described process, a luminance signal outputted from an image sensor may be passed through the LPF and sampled at a predetermined sampling rate. Such a sampling operation may also be implemented with the reading operation of the image sensor. If this is done, the hardware construction of the camera can be greatly simplified and highly accurate offset sampling can be achieved.
However, the aforesaid arrangement encounters the problem of the sampling frequency fso of the image sensor.
In general, if compatibility is to be retained with respect to various kinds of CHSV cameras or reproducing apparatus, the sampling frequency fso for sampling pixels needs to be fixed.
However, if the reading operation of the image sensor is utilized as a sampling operation, the sampling frequency fso is determined by the sampling rate of each individual solid-state image sensor. As a result, the sampling frequency fso will arbitrarily vary with the kind of image sensor used.
The above-described problem restricts the freedom of selection of solid-state image sensors. This restriction is serious when sensors developed for other kinds of devices are to be utilized.